The present invention relates to communication networks that obtain multiple transmitters accessing a shared medium such as a frequency channel or frequency band, in a time division manner. Such channel access methods, usually called Time Division Multiple Access (TDMA), are very popular among satellite communication networks, as well as cellular networks and even with landline communications protocols.
When multiple users access a shared communications channel, and occasionally several signals transmitted by several users simultaneously arrive at a single receiver, such signals might collide or interfere with each other, and reduce receiving success probability. Clearly, such transmission collisions decrease the channel capacity, i.e. the number of users that can share this channel at a certain quality of service.
Transmission collisions become a significant issue in TDMA systems, as the number of transmitters and volume of traffic increase, per allocated transmission time and channel bandwidth, and particularly when transmitters are unable to communicate with each other and cannot synchronize or mutually coordinate their transmission timing.
A typical type of such TDMA systems is related to distress radio beacons communicating with satellites. Such wireless transmitters may be deployed in large quantities, all over the world, yet usually share a narrowband channel or a limited set of frequencies. Further, these distress beacons are activated upon local triggering and their transmissions are not synchronized in time with each other, since cannot communicate with each other and do not necessarily obtain a precise time reference. As such radio beacons are most of the time off, typically for several years, and transmit only in rare occasions, for a short time, normally in periodic bursts for several days, a narrowband frequency channel can basically be shared by these beacons. However, as tens and hundreds of thousands of such beacons are deployed, sharing a narrowband operating channel, simultaneous transmissions might statistically occur, interfering with each other and decreasing the probability of a distress message to be detected. Then, in order to ensure a certain quality of service, i.e. a minimal probability for a distress message to be detected within a specific period of time, the number of transmitters per channel should be limited. Obviously, such a system could be more efficient in exploitation of the allocated spectrum, if channel capacity could be increased, e.g. by reducing transmission collision rate.
One particular TDMA satellite system, COSPAS-SARSAT, is specifically discussed hereby. Focusing on this specific case is done for clarification purposes and is not to limit the scope of the invention.
Cospas-Sarsat is a satellite communications system to assist Search and Rescue (SAR) of people in distress, all over the world and at anytime. The system was launched in 1982 by the USA, Canada, France and the Soviet Union (now Russia) and since then, it has been used for thousands of SAR events and has been instrumental in the rescue of over 20,000 lives worldwide. The goal of the system is to detect and locate signals of distress radio beacons in order to support all organizations in the world with responsibility for SAR operations, whether at sea, in the air or on land. The system uses spacecraft—Low Earth Orbit (LEO); Medium Earth Orbit (MEO)—in the future; and Geostationary (GEO) satellites, and ground facilities, to detect and locate compatible radio beacons. Cospas-Sarsat radio beacons operate in the 406 MHz band (and 121.5 MHz until 2009). The position of the beacon is determined either by the Doppler shift of the received beacon signal or by position data provided by an embedded Global Navigation Satellite System (GNSS) decoder (i.e. receiver), integrated with the radio beacon. The radio beacon location and other related data are then forwarded from satellites to the proper shore stations via the Cospas-Sarsat network. A detailed description of the Cospas-Sarsat System is provided in the document “Introduction to the Cospas-Sarsat System, C/S G.003”—http://cospas-sarsat.org/Documents/gDocs.htm
Numerous present and future GNSS decoders can be considered to be integrated with Cospas-Sarsat radio beacons, mainly the presently operative US Global Positioning System (GPS), but also the Russian GLONASS (or GLONAS), the upcoming European Galileo and the planed Chinese COMPASS and Indian IRNSS.
Several types of Cospas-Sarsat beacons are approved for use, differing mainly in their mechanical structure and activation method, customized for different applications: a) Emergency Position Indicating Radio Beacon (EPIRB) for marine use; b) Emergency Locator Transmitter (ELT) for aviation use; and c) Personal Locator Beacon (PLB) for personal use. PLBs are popular for terrestrial use, by hikers, skiers, hunters and travelers, in addition to marine utilization.
Cospas-Sarsat radio beacons are independent transmitters, totally disconnected from each other; however share a common narrow frequency band. The total bandwidth allocated for Cospas-Sarsat is 100 KHz (406.0-406.1 MHz), divided to 3 KHz bandwidth channels. Normally, each beacon is factory set to one of these channels, not configured to be changed in the field. Then, each channel (theoretically 33 channels, however practically much less mainly due to Doppler shift limitations and system overhead) is shared by tens or hundreds of thousands of beacons, considering that a beacon is most of the time off. When activated (automatically or manually), a Cospas-Sarsat beacon transmits short bursts, each one approximately 0.5 seconds long, every 50 seconds, for several days, until its battery drains. In order to avoid repetitive collisions between two active beacons, a beacon is required to set its transmission cycle to 50+/−2.5 seconds, and the period should be randomized around a mean value of 50 seconds, so that time intervals between transmissions are randomly distributed on the interval 47.5 to 52.5 seconds. Cospas-Sarsat designers calculated the channel capacity based on the population of beacons per channel (e.g. 200K), rate of activation (e.g. 1:100K), activity period (e.g. 0.5 seconds every 50 seconds for 7 days) and the required detection probability (e.g. 95% to detect a distress message within 5 minutes). A comprehensive analysis of the system parameters and performance can be found through the following links    http://cospas-sarsat.com/DocumentsTSeries/T12Nov1 07 CompleteDocPart1.pdf    http://cospas-sarsat.com/DocumentsTSeries/T12Nov1 07 CompleteDocPart2.pdf
A simple calculation shows that the chance of two Cospas-Sarsat beacons transmissions to collide is roughly 2% (assuming detection by same receiver); provided that each transmission is 0.5 seconds long and transmission cycle is 50 seconds. This calculation is based on the observation that for any first beacon transmission, a second beacon transmission will collide (even partially) with, if it starts between 0.5 seconds before to 0.5 seconds after the first transmission starts, i.e. have (0.5+0.5)/50=1:50 collision probability.
This collision probability of two Cospas-Sarsat active beacons could be significantly reduced, applying a time slotted transmission method. For example, if the transmission cycle of 50 seconds is divided to 100 adjacent time slots of 0.5 seconds each, and if transmissions take not more than 0.5 seconds, then for any first beacon transmission, a second beacon transmission will collide with, only if using the same time slot, i.e. achieve a collision probability of about 1:100.
This technique is a well known variation of a TDMA communications method called ALOHA. ALOHA is a simple communications scheme in which each transmitter in a network sends data whenever there is a frame to send. If the frame is successfully received, the next frame is sent. If the frame fails to be received at the destination, it is sent again. This protocol was originally developed at the University of Hawaii for use with satellite communications in the Pacific, among remote deployed devices. An improvement to the original Aloha protocol was Slotted Aloha, which introduced discrete timeslots for transmission and doubled the maximum Aloha throughput.
Yet, such a slotted method requires careful time synchronization among transmitters. For this purpose, an accurate Time of Day (TOD) reference could be provided for all transmitters by a GNSS such as GPS. So it is disclosed by U.S. Pat. No. 7,139,258 to Tillotson. Yet, Tillotson relates to a specific case where the duration of RF transmitted bursts is shorter than the transmission propagation time between said transmitters. This is usually not the case with many systems, particularly based on satellites, such as Cospas-Sarsat, where burst duration is about 500 milliseconds but propagation time is about 120 milliseconds (for most distant satellites—Geostationary).
Still an important issue to consider is the propagation time (or propagation delay) variation, caused by the variety of distances between different transmitters to one receiver. Such difference can cause signals transmitted from two beacons, originally separated in time, to overlap and collide at the receiver due to the longer time delay of the signal that traveled a longer distance. For example, if two remote transmitters are detected by the same satellite receiver, but one transmitter is closer to the receiver than the second transmitter, by about 6,400 Kms (roughly the radius of earth), then the first signal will travel about 22 milliseconds less. Since such beacons are deployed worldwide, and can be placed anywhere on earth, and since most of the receiving satellites (all excluding the Geostationary) are constantly moving relatively to earth, it is quite difficult to control the propagation delay variation in such a satellite based network.
A further correlation can be found between transmission collisions and the location of transmitters relatively to each other, considering a worldwide scale. For example, if two active transmitters are located far away from each other, e.g. on Antipodes, there is a considerable chance that each of these transmissions will be detected by a different satellite, thus will not interfere with each other. Further, this characteristic can be utilized as done in cellular networks, i.e. divide the earth surface to several cells and reuse the same channels and/or timeslots in geographically distant cells. Yet, communication isolation based on geographical separation is more complicated to achieve in satellite based networks than in terrestrial cellular networks. Actually, satellite based networks are often designed to provide a wide geographical coverage, which contradicts with the cellular concept of wireless communication isolation based on geographical separation. For example, a geostationary satellite as employed in systems as Cospas-Sarsat or Inmarsat can cover about one third of the earth surface. However Cospas-Sarsat employs also LEOs and in the near future its payloads will be installed onboard MEOs (Galileo and GPS), which have a smaller footprint on earth surface and may better serve for cellular communication methods related to transmitters located on earth surface.
U.S. Patent Application 20070133592 to Zheng discloses a method for time slot scheduling in a wireless TDMA mesh network based on spatial orientation of the network nodes. Yet this method requires nodes to communicate with each other.
U.S. Pat. No. 5,838,674 to Forssen discloses circuitry and method for TDMA communication system, employing a plurality of time slots, enabling concurrently transmitting at least two signals by two stations spatially spaced apart from one another and positioned at any location within a selected area. However, this method requires a plurality of antennas at each of said stations, and an antenna pattern former to indicate the positioning of said transmitting stations.
U.S. Pat. No. 7,304,963 to Amouris discloses a method and system for dynamically allocating a set of broadcast TDMA channels to a network of transceiver nodes. This method is based on timeslot partitioning and geographic location, yet, it deals with a network of transceiver nodes, i.e. nodes capable of receiving, as well as transmitting, and nodes that share not a single channel but a plurality of channels. Further, for a specific time slot and specific positioning coordinates, this method allocates a unique channel, thus, if would not avoid a communications conflict between nearby nodes.
U.S. Pat. No. 7,082,111 to Amouris discloses a method and system for dynamically allocating time slots of a common TDMA broadcast channel to a network of transceiver nodes. This invention allocates time slots to TDMA devices according to their geographical position, yet it is adapted to nodes capable of receiving. Accordingly, this method requires nodes to receive data in order to resolve time slot allocation conflicts occurring when two transceivers are in close distance. In addition, Amouris does not refer to the length of time slots, neither to propagation delay issues that might be significant in satellite based networks. Furthermore, this invention does not teach or suggest using a GNSS or GPS for accurate timing and location determination.
U.S. Pat. No. 6,115,371 to Berstis discloses a satellite uplink separation using time multiplexed global positioning system cell location beacon system. This method, for allocating bandwidth to devices seeking to initiate contact with a communication service, suggests using time slots according to self location determined by GPS. Yet, Berstis' invention is also related to devices capable of receiving. Furthermore, it deals with a preliminary and relatively short phase, in a two-way communications network, which has the limited role of initiating a contact with a communication service, thus, for example, is limited in avoiding transmission collisions between devices placed in the same cell.
The present art methods described above have not yet provided satisfactory solutions to the problem of transmission collisions among TDMA transmitters that share a common one-way communications channel.
It is an object of the present invention to provide a system and method to increase the capacity of a channel shared by TDMA transmitters, by reducing the collision rate among said TDMA transmitters.
It is also an object of the present invention to provide a system and method for reducing the collision rate among remote TDMA transmitters that have no means to communicate with each other so cannot mutually coordinate their transmissions.
It is another object of the present invention to provide a system and method for reducing the collision rate among TDMA transmitters, particularly distress radio beacons communicating with satellite borne receivers.
It is still an object of the present invention to provide a system and method for reducing the collision rate among TDMA transmitters, based on time synchronization and positioning information provided by a GNSS such as GPS or Galileo or GLONASS.
It is yet another object of the present invention to provide a method for improving present or/and planed systems for SAR and in relation to GNSS, such as Cospas-Sarsat and Galileo.
It is still another object of the present invention to provide a system and method for reducing collisions among TDMA transmitters, minimizing cost and size and power consumption of said transmitters.
It is nonetheless an object of the present invention to provide an apparatus and method for reducing collisions among TDMA transmitters, which are part of systems for SAR such as Cospas-Sarsat.
Other objects and advantages of the invention will become apparent as the description proceeds.